This invention relates to feedback control of air-fuel ratio of an internal-combustion engine and, more specifically, to air-fuel ratio feedback control to be performed when deterioration of a lean NOx catalyst is detected.
Japanese Patent Application Unexamined Publication (Kokai) No. H10-299460 discloses a device for determining deterioration of a NOx trapping agent based on outputs of oxygen density sensors disposed upstream and downstream respectively of the NOx trapping agent. More specifically, after air-fuel ratio enrichment is started for releasing NOx from the NOx trapping agent in an exhaust gas purifying device, the prior art device measures a time period required for an output value of an oxygen density sensor disposed downstream of the exhaust gas purifying device to change to a rich air-fuel ratio after an output value of a LAF (linear air-fuel ratio) sensor disposed upstream of the exhaust gas purifying device has changed to a rich air-fuel ratio. The device determines that the NOx trapping agent has deteriorated when the measured time period is less than a predetermined value.
Japanese Patent Application Unexamined Publication (Kokai) No. 2001-107719 discloses a system for determining deterioration of a lean NOx catalyst in an internal-combustion engine which is provided with a three-way catalyst and a lean NOx catalyst in its exhaust system. The system first measures a time period required for an output TVO2 of an O2 sensor disposed downstream of the lean NOx catalyst to change to a value representing a rich air-fuel ratio after an output SVO2 of an O2 sensor disposed upstream of the lean NOx catalyst changed to a value representing a rich air-fuel ratio. The system determines deterioration of the NOx catalyst based on the measured time period.
Besides, Japanese Patent Application Unexamined Publication (Kokai) No. H10-9019 discloses a technique for using an adaptive controller for calculating adaptive parameters representing dynamic characteristics of an internal-combustion engine so as to control air-fuel ratio based on the calculated parameters.
However, in such conventional techniques, if control responsiveness is high when they are used to make a feedback control upon an air-fuel ratio, wrong detection of the NOx catalyst deterioration may take place because actual air-fuel ratio may overshoot when the air-fuel ratio is enriched for determining deterioration of the NOx catalyst.
In order to resolve the above-described problem, the present invention provides an air-fuel ratio feedback control apparatus for an internal-combustion engine. The air-fuel ratio feedback control apparatus according to an aspect of the invention comprises an air-fuel ratio detector that is disposed in an exhaust system of the internal-combustion engine for detecting air-fuel ratio of exhaust gas. The apparatus further comprises a NOx purifier for purifying NOx contained in the exhaust gas during a lean operation and a feedback controller for performing feedback control of an air-fuel ratio of air-fuel mixture to be supplied to the internal-combustion engine based on the output of the air-fuel ratio detector. The air-fuel ratio feedback control apparatus further comprises diagnostic system for diagnosing the NOx purifier by changing the air-fuel ratio of the air-fuel mixture to be supplied to the internal-combustion engine from lean to rich and detecting outputs of an O2 sensor disposed downstream of the NOx purifier. The feedback control apparatus further comprises a gain changer for changing feedback control gain of the feedback controller to a smaller value when the air-fuel ratio is changed from lean to rich by the diagnostic system.
According to an aspect of the present invention, the feedback control gain of the feedback controller is made smaller when the diagnostic device for diagnosing the NOx purifier changes the air-fuel ratio from lean to rich, so that the overshoot of the actual air-fuel ratio can be avoided and detection accuracy in the diagnosis for the NOx purifier can be improved.